Lignocellulosic feedstocks and wastes, such as agricultural residues, wood, forestry wastes, sludge from paper manufacture, and municipal and industrial solid wastes, provide a potentially large renewable feedstock for the production of chemicals, plastics, fuels and feeds. Lignocellulosic feedstocks and wastes, containing the carbohydrate polymers cellulose, and hemicellulose, as well as lignin, are generally treated by a variety of chemical, mechanical and enzymatic means to release primarily hexose and pentose sugars, which can then be fermented to useful products.
Pretreatment methods are used to make the carbohydrate polymers, or polysaccharides, of lignocellulosic biomass more readily accessible to cellulolytic enzymes used in saccharification. A major impediment to cellulolytic enzyme digestion of polysaccharide is the presence of lignin, which is a barrier that limits the access of the enzymes to their substrates, and provides a surface to which the enzymes bind non-productively. In addition, the crystallinity of cellulose microfibrils restricts enzyme access providing an obstacle to saccharification. Pretreatment methods that attempt to overcome these challenges include: steam explosion, hot water, dilute acid, ammonia fiber explosion, alkaline hydrolysis (including ammonia recycled percolation), oxidative delignification, organosolv, and ozonation. Costs of chemicals, chemical recovery, energy inputs, and capital equipment make many methods not amenable to commercial production.
Biomass pretreatment using low amounts of aqueous ammonia and a high solids concentration is disclosed in commonly owned U.S. Pat. No. 7,932,063. US 20080008783 discloses treatment of biomass, which has been ground and which contains varying moisture contents, with anhydrous ammonia in the liquid or vapor state, and/or concentrated ammonia:water mixtures in the liquid or vapor state, to obtain a mixture in which the ratio of ammonia to dry biomass is between about 0.2 to 1 and 1.2 to 1, and the water to dry biomass ratio is between about 0.2 to 1.0 and 1.5 to 1. The temperature is maintained between about 50° C. and 140° C. and the pressure is rapidly released by releasing ammonia from the vessel to form a treated biomass.
Milling, where biomass is ground, has been used as a non-chemical pretreatment, or used in combination with ozonolysis ((Kabeya et al. (1993) Shikoku Kogyo Gijutsu Shikensho Kenkyu Hokoku 24:42-90).
There remains a need for alternative efficient, low cost lignocellulosic biomass pretreatment processes that prepare biomass for saccharification.